1. Field of the Invention
The present invention relates generally to ink-jet hard copy apparatus, and, more specifically, to methods and apparatus for the use of electrostatic devices for detection of ink drop characteristics and printing with correction for offsets.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
FIG. 1 depicts an ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller 102 (usually a microprocessor-controlled printed circuit board) connected by appropriate cabling to a computer (not shown). Cut-sheet print media 105, loaded by the end-user onto an input tray 107, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station where graphical images or alphanumeric text is created. A carriage 109, mounted on a slider 111, scans the print medium. An encoder 113, 114 subsystem is provided for keeping track of the position of the carriage 109 at any given time. A set of ink-jet pens, or print cartridges, 115x (where the letter is a color designation, e.g., cyan (C), magenta(M), yellow (Y), black (K), red (R), blue (B), green (G), or a fixer chemical (F)) are releasably mounted in the carriage 109 for easy access. In pen-type hard copy apparatus, separate, replaceable or refillable, ink reservoirs 117x are located within the housing 103 and appropriately coupled to the pen set 115 via ink conduits 119. Once a printed page is completed, the print medium is ejected onto an output tray 121. Printing is accomplished on the print medium as it transits a print zone 123.
A simplistic schematic of a swath-scanning ink-jet pen 200 is shown in FIG. 2 (PRIOR ART). The body 210 of the pen 200 generally contains an ink accumulator and regulator mechanism 212. The internal accumulator and regulator are fluidically coupled 119 (FIG. 1 only) to an off-axis ink reservoir 117x in any known manner to the state of the art. The printhead 214 element includes an appropriate electrical connections 220 (such as a tape automated bonding flex tape) for transmitting signals to and from the printhead. Columns of individual nozzles 217 form an addressable firing array 216. The typical state of the art scanning pen printhead may have two or more columns with more than one-hundred nozzles per column, each nozzle having a diameter of about {fraction (1/300)}th inch or less. Multi-color pens having the nozzle array 216 is subdivided into discrete subsets, known as xe2x80x9cprimitivesxe2x80x9d are also known in the art. In a thermal ink-jet pen, the drop generator includes a heater resistor subjacent each nozzle which on command superheats local ink to a cavitation point such that an ink bubble""s expansion and collapse ejects a droplet from the associated nozzle 217. In commercially available products, piezoelectric and wave generating element techniques are also used to fire the ink drops. Degradation or complete failure of the drop generator elements cause drop volume variation, trajectory error, or misprints, referred to generically as xe2x80x9cartifacts,xe2x80x9d and thus affect print quality.
In essence, the ink-jet printing process involves digitized dot-matrix manipulation of drops of ink, or other liquid colorant, ejected from a pen onto an adjacent print media. [For convenience of describing the ink-jet technology and the present invention hereinafter, all types of print media are referred to simply as xe2x80x9cpaper,xe2x80x9d all compositions of colorants are referred to simply as xe2x80x9cink,xe2x80x9d and all types of hard copy apparatus are referred to simply as a xe2x80x9cprinter.xe2x80x9d No limitation on the scope of invention is intended nor should any be implied.] Each column or selected subset of nozzles selectively fires ink droplets (typically each being only a few picoliters in liquid volume, having a nominal diameter of only about ten in flight and forming a dot of approximately forty xcexcm on the paper) that create a predetermined print matrix of dots on the adjacently positioned paper as the pen is scanned. The pen scanning axis is the x-axis, the paper path is the y-axis and the ink drop firing direction is the z-axis; related linear offsets are referred to as delta-x, delta-y and delta-z, respectively, and rotational offsets are referred to as theta-x (printhead planar pitch), theta-y (roll) and theta-z (yaw). A given nozzle of the printhead is used to address a given matrix column print position on the paper (referred to as a picture element, or xe2x80x9cpixelxe2x80x9d). Horizontal positions, matrix pixel rows, on the paper are addressed by repeatedly firing a given nozzle at matrix row print positions as the pen is scanned. Thus, a single sweep scan of the pen across the paper can print a swath of tens of thousands of dots. The paper is stepped to permit a series of contiguous swaths. Complex digital dot matrix manipulation is used to render alphanumeric characters, graphical images, and even photographic reproductions from the ink drops. Page-wide ink-jet printheads are also contemplated and are adaptable to the present invention.
As can now be recognized, the seemingly simple process of creating a computer print by scanning a plurality of printheads while actively firing minute ink droplets across a z-axis gap onto a sheet of paper as a digital dot matrix of organized pixels in order to form sophisticated graphics and photographs is actually a highly complex process. The reduction of visible artifacts in the print is a constant concern of the system designer.
A variety of techniques have been used over the years since the inception of ink-jet printing to ensure appropriate dot placement. In U.S. Pat. No. 4,794,411, filed in 1987 by Taub et al., a THERMAL INK-JET HEAD STRUCTURE WITH ORIFICE OFFSET FROM RESISTOR methodology teaches a controlling of misdirection of fired drops by proper nozzle design. In U.S. Pat. No. 4,922,268, filed in 1989 by Osborne, a PIEZOELECTRIC DETECTOR FOR DROP POSITION DETERMINATION IN MULTI-PEN THERMAL INK JET PEN PRINTING SYSTEMS teaches a methodology for mapping the positions of nozzles with respect to a pattern of openings in the detector [U.S. Pat. No. 5,036,340 filed in 1990 by Osborne is a continuation-in-part of ""268.] In U.S. Pat. No. 4,922,270 filed simultaneously with Osborne by Cobbs et al., an optical or piezoelectric or electrostatic phase plate detector through which a drop is fired and measurements are used for INTER PEN OFFSET DETERMINATION AND COMPENSATION IN MULTI-PEN THERMAL INK JET PEN PRINTING SYSTEMS [U.S. Pat. No. 5,109,239 is a continuation-in-part of ""270]. In U.S. Pat. No. 5,404,020, filed in 1993 Cobbs teaches a PHASE PLATE DESIGN FOR ALIGNING MULTIPLE INKJET CARTRIDGES BY SCANNING A REFERENCE PATTERN. In U.S. Pat. No. 5,448,269, filed in 1993 by Beauchamp et al., MULTIPLE INKJET CARTRIDGE ALIGNMENT FOR BIDIRECTIONAL PRINTING BY SCANNING A REFERENCE PATTERN is shown. In U.S. Pat. No. 5,835,108, filed in 1996, Beauchamp et al. teach a CALIBRATION TECHNIQUE FOR MISDIRECTED INKJET PRINTHEAD NOZZLES. Each of the aforementioned patents is assigned to the common assignee herein and incorporated herein by reference.
As thermal ink-jet pens are used, damage may occur, such as due to a printhead crash against the adjacent paper, resistor burn-out, ink cogation, and the like as is known to those skilled in the art, causing drop characteristic changes and trajectory changes. Ink drop trajectory can change as a print is being rendered due to ink puddling around the nozzle orifice. Frequent servicing of the printhead, such as by spitting into a waste ink collector or wiping at a service station, degrades throughput. Moreover such wiping of the printhead can wear the nozzle plate which can cause trajectory errors. Thus, while pen xe2x80x9chealthxe2x80x9d is a constant concern, optimally, a pen should only be serviced if and when it is required.
Other techniques related to the actual pixel printing, such as error diffusion, resolution synthesis, or other printing mode digital manipulation are also employed to reduce the number or visibility of print artifacts.
No technique appears to be available for exact printing plane ink drop trajectory determination during printing. Therefore, a method and apparatus is needed to verify each nozzle operation during a print job without impacting the speed of the print job. The method and apparatus should characterize the entire pen swath height in one or two passes.
In a basic aspect, the present invention provides a method for detecting scanning ink-jet printhead drop firing characteristics, including the steps of: determining a set of drop generators of the printhead to be used in a next printing scan from a predetermined set of data; firing selected drop generators at a detector fixedly located within a printing zone of the printhead, the detector having a matrix of detecting elements sized substantially identically to pixels to be printed wherein the elements are arranged in a like plane and in like orientation as the pixels to be printed; and determining ink drop firing characteristics as a function of a correlation of the set of data to a second set of data produced by the detecting elements receiving drops of ink from the selected drop generators.
In another basic aspect, the present invention provides a method of printing with a set of scanning ink-jet printheads, including the steps of: receiving a first set of data indicative of a printed image to be rendered; parsing the data into swaths subsets; determining printhead nozzle firing requirements for a next swath to be printed; prior to printing the next swath on a sheet of print media, firing nozzles determined as required for the next swath at a drop detection target located within a print zone of the printheads and having a matrix of detecting elements sized as a function of size of pixels to be printed wherein the elements are arranged in a like plane and in like orientation as the pixels to be printed and located adjacently to the sheet of print media in the print zone; based upon detecting elements struck by drops from fired nozzles, determining if any of the fired nozzles is malfunctioning and based upon detecting elements struck by drops from fired nozzles, determining if any of the fired nozzles has a firing trajectory error; and correcting for any detected malfunctioning nozzles and any firing trajectory errors prior to printing the next swath.
In another basic aspect, the present invention provides an ink-jet hard copy apparatus, having a printing zone and mechanisms for transporting print media to and through the printing zone, including: at least one scanning printhead mechanism for scanning across the printing zone, including scanning across the print media width and an additional predetermined region of the printing zone adjacent to the print media transported thereto, each printhead mechanism having a plurality of individually selectable ink drop generators; an ink drop detection target mechanisms for receiving individual ink drops fired from individually selected ink drop generators, the target mechanisms being mounted in the printing zone in the additional predetermined region; associated with the target mechanisms, mechanisms for determining individual drop detector malfunctions and firing trajectory errors; and associated with the mechanisms for determining individual drop detector malfunctions and firing trajectory errors, mechanisms for providing signals indicative of the individual drop detector malfunctions and firing trajectory errors.
In another basic aspect, the present invention provides a method for detecting and correcting ink drop firing misalignments including the steps of: placing an ink drop detector in a printing zone plane of an ink-jet apparatus; firing ink drops from known position and known timing using a predetermined firing pattern at a predetermined pattern of detector mechanisms for providing signals indicative of position and timing of dots formed by the ink drops on the detector mechanisms; comparing the signals to the known position and known timing; and deriving ink drop firing correction signals based on the step of comparing.
Some of the advantage of the present invention are:
it provides a real time method and apparatus for characterizing ink drop trajectories and alignments of an ink-jet printhead;
it provides an apparatus scalable to a plurality of printheads and a variety of print zone designs;
it provides an apparatus for detecting inter-pen and intra-pen offsets;
it additionally provides a drop volume characterization technique;
in a first embodiment, the detector device can be produced by current microcircuit fabrication technology;
in the first embodiment, signal processing circuits can be incorporated into the silicon die used for a drop detector;
in the first embodiment, detector devices can be scaled for specific implementations using known manner integrated circuit fabrication technology; and
in a second embodiment, detector devices are fabricated using simple, cost-efficient, printed circuit technology.
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprize the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.